a) Field of the Invention
The invention is directed to an arrangement and a method for imaging depth-structured objects and for calibrating the imaging in connection therewith, preferably for generating stereoscopic images from different observation planes at different depth levels of an object.
b) Description of the Related Art
Particularly when imaging depth-structured objects with a non-telecentric entrance pupil position, different depths are imaged with different imaging scales. Further, asymmetric pupil positions can lead to lateral effects. Special measurement systems use telecentric beam delivery to avoid this effect. Knowledge of the distortion characteristic and displacement characteristic in such systems with non-telecentric entrance pupil position is necessary for managing measurement tasks and for the superposition of images from different depths. This information must be gained from test recordings of known structures.
One problem in this respect consists in the blurriness of the images which is caused by defocusing and which is directly related to the different distance between object planes. Shifting of the image position and distortion are defined by the bundle shaping of imaging beams proceeding from an object point through the aperture diaphragm. The unification of the imaging beams in the image plane is disrupted by defocusing, and the blurry images make it more difficult to determine the image positions and, therefore, the parameters for calibration.
It is known to use stereo microscopes to obtain three-dimensional image information from an object to be observed. For this purpose, two-dimensional images are recorded stepwise from different focus planes. The stack of two-dimensional images formed in this way contains the three-dimensional image information of the object.
However, for the reasons mentioned above, the images are distorted in an unwanted manner when passing through the optical systems. The degree of this distortion depends on the geometric-optical imaging characteristics of the objective and the subsequent imaging system, on the size of the lateral offset between the optical axis of the objective and the subsequent imaging system, and on manufacturing defects.
The design-dependent systematic distortion can be canceled out almost completely by mathematical methods. The distortion can be characterized by evaluating the changes in the lateral image coordinates of objects with images neighboring one another with respect to depth, that is, images which are recorded with different object focusing. However, the accuracy of this evaluation is impaired and limited by the blurriness that is inevitably associated with refocusing.
Further, it is known to solve the problem of preventing the energy loss that is caused in stereo-microscope recordings of fluorescence images by color cameras by recording a series of images with a monochrome camera and superimposing the images of this series additively and with colorization. However, the residual errors of the longitudinal chromatic aberration results as blurriness in individual images due to defocusing.
In devices of known construction, compensation of blurriness by refocusing the stereo microscope inevitably leads to a lateral shift and to a change in the lateral image distances. If this influence is corrected by image processing, portions of the image for superposition are lost due to the lateral shift. Mechanical compensation by displacement of the object and adaptation of the imaging scale is complicated.
It is desirable to eliminate this negative effect through suitable possibilities for focus correction.